The proteasome (also known as macropain, the multicatalytic protease, and 20S protease) is a high molecular weight, multisubunit protease which has been identified in every examined species from an archaebacterium to human. The enzyme has a native molecular weight of approximately 650,000 and, as revealed by electron microscopy, a distinctive cylinder-shaped morphology (Rivett, (1989) Arch. Biochem. Biophys. 268:1-8; and Orlowski, (1990) Biochemistry 29:10289-10297). The proteasome subunits range in molecular weight from 20,000 to 35,000, and are homologous to one another but not to any other known protease.
The 20S proteasome is a 700 kDa cylindrical-shaped multicatalytic protease complex comprised of 28 subunits, classified as α- and β-type, that are arranged in 4 stacked heptameric rings. In yeast and other eukaryotes, 7 different a subunits form the outer rings and 7 different β subunits comprise the inner rings. The a subunits serve as binding sites for the 19S (PA700) and 1 IS (PA28) regulatory complexes, as well as a physical barrier for the inner proteolytic chamber formed by the two β subunit rings. Thus, in vivo, the proteasome is believed to exist as a 26S particle (“the 26S proteasome”). In vivo experiments have shown that inhibition of the 20S form of the proteasome can be readily correlated to inhibition of 26S proteasome.
Cleavage of amino-terminal prosequences of β subunits during particle formation exposes amino-terminal threonine residues, which serve as the catalytic nucleophiles. The subunits responsible for catalytic activity in proteasome thus possess an amino terminal nucleophilic residue, and these subunits belong to the family of N-terminal nucleophile (Ntn) hydrolases (where the nucleophilic N-terminal residue is, for example, Cys, Ser, Thr, and other nucleophilic moieties). This family includes, for example, penicillin G acylase (PGA), penicillin V acylase (PVA), glutamine PRPP amidotransferase (GAT), and bacterial glycosylasparaginase. In addition to the ubiquitously expressed β subunits, higher vertebrates also possess three interferon-γ-inducible β subunits (LMP7, LMP2 and MECLI), which replace their normal counterparts, β5, β1 and β2, respectively. When all three IFN-γ-inducible subunits are present, the proteasome is referred to as an “immunoproteasome”. Thus, eukaryotic cells can possess two forms of proteasomes in varying ratios.
Through the use of different peptide substrates, three major proteolytic activities have been defined for the eukaryote 20S proteasomes: chymotrypsin-like activity (CT-L), which cleaves after large hydrophobic residues; trypsin-like activity (T-L), which cleaves after basic residues; and peptidylglutamyl peptide hydrolyzing activity (PGPH), which cleaves after acidic residues. Two additional less characterized activities have also been ascribed to the proteasome: BrAAP activity, which cleaves after branched-chain amino acids; and SNAAP activity, which cleaves after small neutral amino acids. Although both forms of the proteasome possess all five enzymatic activities, differences in the extent of the activities between the forms have been described based on specific substrates. For both forms of the proteasome, the major proteasome proteolytic activities appear to be contributed by different catalytic sites within the 20S core.
In eukaryotes, protein degradation is predominately mediated through the ubiquitin pathway in which proteins targeted for destruction are ligated to the 76 amino acid polypeptide ubiquitin. Once targeted, ubiquitinated proteins then serve as substrates for the 26S proteasome, which cleaves proteins into short peptides through the action of its three major proteolytic activities. While having a general function in intracellular protein turnover, proteasome-mediated degradation also plays a key role in many processes such as major histocompatibility complex (MHC) class I presentation, apoptosis and cell viability, antigen processing, NF-κB activation, and transduction of pro-inflammatory signals.
Proteasome activity is high in muscle wasting diseases that involve protein breakdown such as muscular dystrophy, cancer and AIDS. Evidence also suggests a possible role for the proteasome in the processing of antigens for the class I MHC molecules (Goldberg, et al. (1992) Nature 357:375-379).
Proteasomes are involved in neurodegenerative diseases and disorders such as Amyotrophic Lateral Sclerosis (ALS), (J Biol Chem 2003, Allen S et al., Exp Neurol 2005, Puttaparthi k et al.), Sjogren's syndrome (Arthritis & Rheumatism, 2006, Egerer T et al.), systemic lupus erythematoses and lupus nephritis (SLE/LN), (Arthritis & rheuma 2011, Ichikawa et al., J Immunol, 2010, Lang V R et al., Nat Med, 2008, Neubert K et al), glomerulonephritis (J Am Soc nephrol 2011, Bontscho et al.), Rheumatoid Arthritis (Clin Exp Rheumatol, 2009, Van der Heiden J W et al.), Inflammatory bowel disease (IBD), ulcerative colitis, Crohn's diseases, (Gut 2010, Schmidt N et al., J Immunol 2010, Basler M et al., Clin Exp Immunol, 2009, Inoue S et al.), multiple sclerosis (Eur J Immunol 2008, Fissolo N et al., J Mol Med 2003, Elliott P J et al., J Neuroimmunol 2001, Hosseini et al., J Autoimmun 2000, Vanderlugt C L et al.), Amyotrophic lateral sclerosis (ALS), (Exp Neurol 2005, Puttaparthi k et al., J Biol Chem 2003, Allen S et al.), osteoarthritis (Pain 2011, Ahmed s et al., Biomed Mater Eng 2008, Etienne S et al.), Atherosclerosis (J Cardiovasc Pharmacol 2010, Feng B et al., Psoriasis (Genes & Immunity, 2007, Kramer U et al.), Myasthenia Gravis (J Immunol, 2011, Gomez A M et al.), Dermal fibrosis (Thorax 2011, Mutlu G M et al., Inflammation 2011, Koca S S et al., Faseb J 2006, Fineschi S et al.), renal fibrosis (Nephrology 2011 Sakairi T et al.), cardiac fibrosis (Biochem Pharmacol 2011, Ma y et al.,) Liver fibrosis (Am J Physiol gastrointest Liver Physiol 2006, Anan A et al.), Lung fibrosis (Faseb J 2006, Fineschi S et al et al.), Immunogloulin A nephropathy (IGa nephropathy), (Kidney Int, 2009, Coppo R et al.), Vasculitis (J Am Soc nephrol 2011, Bontscho et al.), Transplant rejection (Nephrol Dial transplant 2011, Waiser J et al.), Hematological malignancies (Br J Haematol 2011, Singh A V et al., Curr Cancer Drug Target 2011, Chen D et al.) and asthma.
Yet, it should be noted that commercially available proteasome inhibitors inhibit both the constitutive and immuno-forms of the proteasome. Even bortezomib, the FDA-approved proteasome inhibitor for the treatment of relapsed multiple myeloma patients, does not distinguish between the two forms (Altun et al., Cancer Res 65:7896, 2005). Furthermore, the use of Bortezomib is associated with a treatment-emergent, painful peripheral neuropathy (PN), this bortezomib-induced neurodegeneration in vitro occurs via a proteasome-independent mechanism and that bortezomib inhibits several nonproteasomal targets in vitro and in vivo (Clin. Cancer Res, 17(9), May 1, 2011).
In addition to conventional proteasome inhibitors, a novel approach may be to specifically target the hematological-specific immunoproteasome, thereby increasing overall effectiveness and reducing negative off-target effects. It has been shown that immunoproteasome-specific inhibitor, could display enhanced efficiency on cells from a hematologic origin (Curr Cancer Drug Targets, 11(3), March, 2011).
Thus there is a need to provide new proteasome inhibitors that are selective of one specific form of the proteasome. In particular there is a need to provide selective immunoproteasome inhibitors, which could be used as therapeutic agents for the treatment of e.g. SLE or other immune or autoimmune disorders in the context of rheumatoid arthritis. Selective immunoproteasome inhibitors are helpful in order to minimize unwanted side effects mediated by inhibition of the constitutive proteasome or other nonproteasomal targets.
WO 2013/092979 A1 describes boronic acid derivatives, which show selectivity towards the inhibition of the LMP7 activity. However, the extent of selectivity, which is achievable with the described types of compounds, is limited, particularly with respect to the split to the inhibitory activity of the constitutive proteasome.
Unspecific inhibitors of the proteasome and the immunoproteasome like Bortezomib and Carfilzomib have demonstrated their clinical value in the indication of multiple myeloma. Although this unspecific profile, hitting major components in the immunoproteasome as well as the constitutive proteasome, is regarded as beneficial in terms of target inhibition and clinical effectiveness, this unspecific profile limits the clinical applicability of these agents by inducing pronounced side effects like thrombocytopenia, neutropenia as well as peripheral neuropathy. To a certain extent, this side effect profile could be attributed to the broad inhibition of the catalytic activity, especially the combined inhibition of the 915 subunits of the constitutive and the immunoproteasome. The approach to come up with more selective inhibitors of the immunoproteasome (and especially the ß5i subunit of the immunoproteasome), in order to reduce major side effects has been described e.g. in 2011 by Singh et al. (Br. J. Hematology 152(2): 155-163) for PR-924, a 100 fold selective inhibitor of the LMP7 subunit of the immunoproteasome. The authors demonstrated the presence of high expression levels of the immunoproteasome in multiple myeloma. The authors also described the effect of a selective inhibitor of the LMP7 subunit on the induction of cell death in MM cell lines as well as CD138+MM primary patient cells without decreasing the viability of control PBMC's of healthy volunteers which can be regarded as a conceptual proof. Besides the concept of a reduced side effect profile for selective 135i inhibitors other groups demonstrated the efficacy of selective 135i inhibition on the viability of Bortezomib resistant cell lines underlining the value and potential perspective for the application of selective LMP7 inhibitors for hematological malignancies (D. Niewerth et al./Biochemical Pharmacology 89 (2014) 43-51).
Surprisingly, it was found that amino boronic acid derivatives according to this invention also inhibit LMP7. These compounds show very good properties in terms of their use in the treatment and/or prevention of medical conditions affected by immunoproteasome activity. In particular the compounds of the present invention are able to inhibit the activity of the immunoproteasome (LMP7) providing a significant split to the inhibitory activity of the constitutive proteasome. Besides this, the structural assembly of the compounds allows a simple and straightforward fine-tuning of the compound properties. Further important advantages are their good results regarding plasma-protein binding, CYP inhibition, PK profile and oral bioavailability.